This paper researches on laser damage characteristics of Ⅱ-type DKDP crystals used for nonlinear frequency conversion of third harmonic under 355 nm laser with 35 ps, 850 ps and 7.6 ns pulse duration. The damage threshold, probability and the damage pinpoints morphology, density, size under different pulse duration are compared and analyzed. The results show that the laser energy absorption of the precursors is linearly related to the pulse duration. Under 35 ps laser, one type of precursors absorbs laser energy, which results in melting damage pinpoints. Under 850 ps laser, two types of precursors absorb laser energy and generate mechanical response form damaged pinpoints made of melting center point and surrounded by cracks. Under 7.6 ns laser, only one type of precursors takes effect and shares the same pinpoints characteristics with 850 ps laser.